For Boeing the trailing three characters designate the aircraft type and operator who ordered the aircraft.
eg: B737-838 is 800 series ordered by Qantas (38)
B737-8FE is 800 series ordered by VA (FE)
B737-476 is 400 series ordered by Australian Airlines (TAA)
For more info:
http://en.wikipedia.org/wiki/List_of...customer_codes